Although benzodiazepines (BZs) are considered to be among the safest prescription drugs in modern medicine, their utility is constrained by a number of side effects, including the liability for dependence, and recent epidemiological research suggests BZ abuse is on the rise in the U.S. The overall goal of this application is to investigate the extent to which GABAA receptor subtypes are differentially involved in tolerance and physical dependence associated with long-term BZ exposure, using relevant nonhuman primate models. This new application builds on research that has implicated unique roles for ?1, ?2, ?3, and ?5 subunit-containing GABAA receptors (?1GABAA, ?2GABAA, ?3GABAA ?5GABAA receptors, respectively) in the abuse-related and therapeutic effects of BZs. As part of a previous grant, we obtained several key preliminary findings: (1) tolerance may develop rapidly to behavioral effects associated with ?1GABAA receptors, but not ?2/3GABAA receptors; and (2) the ability of BZ-type drugs to induce physical dependence may involve ?1GABAA receptors. Together, these findings support the working hypothesis that ?1GABAA receptors are key mediators of both tolerance and physical dependence associated with chronic exposure to BZ-type drugs. However, the extent to which ?2GABAA, ?3GABAA, and ?5GABAA receptors are involved in tolerance and physical dependence remains unclear. In Specific Aim 1 we will evaluate the extent to which tolerance and physical dependence will be induced by chronic treatments with the sedative/anxiolytic BZ, alprazolam (Xanax), the most commonly-abused BZ. Following 30 days of exposure, we will evaluate differential tolerance to behavioral effects, including self-administration, as well as assess spontaneous and precipitated withdrawal. In Specific Aim 2, we will evaluate the hypothesis that ?1GABAA and ?5GABAA receptor subtypes mediate tolerance, whereas ?1GABAA receptor subtypes mediate physical dependence induced by BZs. Using the parameters established in Aim 1, we will conduct studies with a novel series of ligands that are agonists or antagonists with selectivity for ?1GABAA, ?2GABAA, ?3GABAA, and ?5GABAA subtypes. We hypothesize that tolerance development requires co-activation of both the ?1GABAA and ?5GABAA receptor subtypes and that physical dependence involves the ?1GABAA receptor subtype. The research in this new application addresses a key topic for public health by systematically exploring the role that receptor subtypes play in the behavioral effects of chronic exposure to BZ-type drugs. Innovation of this proposal is in (1) the use of rigorous quantitative behavioral models of chronic physical dependence in a species with high translational validity (rhesus macaque); and (2) the use of first-in-kind BZ ligands with unique subtype selectivity.